A transmission of countershaft design intended for longitudinal fitting usually comprises an input shaft, at least one countershaft and an output shaft. The input shaft can be connected to and disconnected from the driveshaft of the drive engine by an engine clutch that acts as a starting and shifting clutch. The countershaft is arranged axis-parallel to the input shaft and is in permanent driving connection therewith by way of an input constant usually formed by a spur gearwheel pair with two fixed wheels arranged in a rotationally fixed manner on the respective transmission shaft (the input shaft and the countershaft). The output shaft is arranged axis-parallel to the countershaft and coaxially with the input shaft, and can be connected selectively to the countershaft by way of a number of gears with different gear ratios. The gears are usually formed as spur gearwheel pairs, each with a fixed wheel arranged in a rotationally fixed manner on one of the transmission shafts (countershaft or output shaft) and a loose wheel mounted to rotate on the other transmission shaft (output shaft or countershaft). To engage a gear, i.e. to produce a drive connection between the countershaft and the output shaft with the gear ratio of the gear concerned, a gear clutch is associated with each loose wheel. The loose wheels of adjacent gears are usually arranged at least in pairs on the same transmission shaft, so that the gear clutches are correspondingly combined in pairs in shifting packets, each with a common shifting sleeve.
The shifting sequence for an upshift from a gear under load to a higher target gear generally begins when the torque delivered by the drive engine is reduced and at approximately the same time the engine clutch is opened, before the loaded gear is disengaged. Thereafter the target gear is synchronized, in that the input speed, i.e. the speed on the input side of the gear clutch of the target gear, which is determined by the speed of the input shaft or the countershaft, is reduced to the synchronous speed on the output side of the gear clutch of the target gear, determined by the speed of the output shaft. Then the target gear is engaged and after that, at approximately the same time, the engine clutch is closed and the torque delivered by the drive engine is increased again. In automated transmissions the input speed is usually detected by means of a speed sensor arranged on the input shaft whereas the output speed is detected by a speed sensor arranged on the output shaft. For the two speeds to be comparable it is necessary to relate them to a single transmission shaft, i.e. to convert them correspondingly. However, since particularly with an arrangement of the loose wheels on the countershaft and the output shaft alternating in pairs it would be relatively complicated to convert the respective speeds at the relevant transmission shaft for the gear clutch of the target gear concerned, it is usual to refer the two speeds, in each case independently of the arrangement of the loose wheel concerned, uniformly to the same transmission shaft, preferably the input shaft. For this it is only necessary to convert the output speed detected at the output shaft, by multiplication by the gear ratio of the target gear and the gear ratio of the input constant, to the input shaft, whereas the input speed detected at the input shaft can remain unchanged. In this case the conversion of the speeds, known per se, will not be dealt with explicitly; rather, the input speed and the output speed are respectively to be understood as the speeds already related to a common transmission shaft, in particular the input shaft.
In general, compared with gear clutches synchronized by means of friction rings and locking gear teeth, unsynchronized gear clutches of the type known as claw clutches have a much simpler structure, lower production costs, more compact dimensions and substantially less susceptibility to wear and defects. In an automated transmission provided with claw clutches, the target gear of an upshift is preferably synchronized by means of a centrally arranged controllable braking device, for example a transmission brake functionally associated with the input shaft or the countershaft. The control of a transmission brake and a shift regulator for synchronizing and engaging an unsynchronized target gear is comparatively simple compared with the control-path-dependent, adjustment-speed and adjustment-force-variable control of a shift regulator for synchronizing and engaging a synchronized target gear, since for this the sensor data from the speed sensors arranged on the input shaft and the output shaft are essentially sufficient.
A typical transmission brake of an automated transmission of countershaft design is described in DE 196 52 916 B4. This known transmission brake is in the form of a hydraulically or pneumatically actuated disk brake arranged on the end of the countershaft on the engine side. The disks of the transmission brake are connected in a rotationally fixed manner by way of inner and outer carrier teeth, alternately to the countershaft and to a brake housing fixed on the transmission housing. The transmission brake is actuated by a piston arranged to move axially in a brake cylinder, which is acted upon axially on the outside by the controllable control pressure in the pressure space of the brake cylinder and thereby pressed against the disks in opposition to the restoring force of a spring arranged between the piston and the countershaft. The control pressure acting in the pressure space is regulated by means of an inlet valve connected on the inlet side to a pressure line and an outlet valve connected on the outlet side to an unpressurized line, which on the outlet side and inlet side are respectively connected in each case by a short duct to the pressure space of the brake cylinder. The two valves can optionally be in the form of 2/2-way magnetic switching valves, which enable simple control sequences and on which the present invention is based, or 2/2-way magnetic timed valves which enable more complex regulation sequences.
DE 103 05 254 A1 describes a method for controlling a transmission brake of such type in an automated variable-speed transmission of countershaft design provided with claw clutches, in which, during the braking of the countershaft associated with an upshift, the number of program cycles or the time taken to reach the synchronous speed is determined having regard to the gradient of the output speed. This takes into account the fact that the synchronous speed determined by the output speed is not a constant, but can decrease during the shift-associated traction or thrust force interruption, for example when driving on an uphill stretch, or increase, for example on a downhill stretch. To determine the synchronous time a so-termed sum gradient is formed, which as the difference between the gradients of the output speed and of the input speed, constitutes a type of effective gradient. However, this known method assumes that when the transmission brake reaches the synchronous point it is disengaged spontaneously, i.e. without any time delay until the beginning of the disengagement process and without any steady reduction of the braking torque, whereas the reality is different.
Accordingly, in an improved method described in DE 103 30 517 A1 a lead time is provided, by virtue of which the response behavior of the control system of the transmission brake and of the transmission brake itself is taken into account. In this method the signal for disengaging the transmission brake is emitted a certain lead time before the determined synchronous time point has been reached. Moreover, in this method it is provided that the quality of upshifts is evaluated in relation to the reaching of a specified target speed window at the moment when the gear clutch of the target gear is engaged, and the lead time is left unchanged, corrected or recalculated as a function of the quality of the upshift concerned. In this known method, however, the steady decrease of the braking torque of the transmission brake during the disengagement process and the influence of operating parameters, such as the operating temperature of the transmission brake, on the response behavior and the disengagement process are not explicitly determined and are consequently taken into account only approximately by the lead time, which results in certain inaccuracy of the method.